The present invention relates to a vibration detector for a rotating shaft, and more particularly, to such a detector used with a Czochralski-type crystal puller.
In the Czochralski-type crystal puller, a melt of the crystal material is disposed in a heated crucible, which is attached to a rotating shaft. Surrounding the heater is an insulating jacket called "heater furniture", and the whole apparatus is mounted on a baseplate. A seed crystal is placed in the melt and pulled up, and some of the melt solidifies on the seed in crystallographic alignment therewith. This solidified portion is called the "boule". When the crystal to be formed is GaAs, a very high pressure inert gas must be used to prevent the As from vaporizing. The high pressure causes the gas to be a good thermal conductor. To prevent loss of heat through the gas, which would occur if the gas goes between the heater furniture and the crucible, tight tolerances are used between the rotating crucible and the heater furniture, in particular an insulating cap thereof. However, then the crucible will sometimes make contact with the cap. If the contact is hard enough, it will cause the crucible to break out into a rotary oscillation. This oscillation can cause failure to gain control of crystal growth, and therefore termination of the pull with shorter than desired crystal length, twinning or dislocations in the crystal before the desired length is achieved, and breaking of the boule off the seed and its falling into melt. If boule breakage occurs, it can fracture the crucible. This often causes catastrophic damage to the puller, and since the leaking melt is conductive and hot, this can result in a destroyed heater and even a partially melted baseplate. Further, all items that come in contact with the melt (except the crucible) become contaminated waste.
Presently it can only be determined if the crucible is in a rotational oscillation by observing it on a video monitor. This is dependent on sufficient heat in the crystal chamber to adequately light the crucible, e.g., at least about 4 hours after start of heat-up. It is also necessary to have an operator present at the time of the oscillation and act promptly (typically within a few seconds) to correct it.
It is therefore an object of the present invention to have a warning system for vibration of rotating shaft that provides clear and early warning of the vibration.